Pulmonary fibrosis refers to a symptom in which lung tissues become fibrotic due to the accumulation of excess collagen and other extracellular matrix. Within a classification of pulmonary fibrosis, idiopathic pulmonary fibrosis is a chronic intractable disease, carrying a poor prognosis with an average median survival time of three years and a five year survival rate of 20 to 40%. For the treatment of pulmonary fibrosis, steroid drugs and immunosuppressants are used; however, no effective therapy which can improve the prognosis is currently available, and thus development of a new therapeutic drug is demanded.
Recently, it has been revealed that there are many diseases whose onset is attributed to a gene, and many genes are also reported to be involved in pulmonary fibrosis (Patent Documents 1 to 4, Non Patent Documents 1 to 6). As the main factor associated with pulmonary fibrosis, TGF-β1 (Patent Documents 1 and 2, Non Patent Documents 2 to 6), Smad3 (Non Patent Document 1), MCP-1 (Patent Document 3), and the like are reported.
Meanwhile, a nucleic acid, particularly, siRNA induces degradation of mRNA of a gene having a sequence identical or almost identical to a specific sequence present in a cell, thereby inhibiting the expression of a target gene (RNA interference). Accordingly, the function of inhibiting the expression of the target gene because of RNA interference is useful for the amelioration or treatment of disease symptoms induced by abnormal expression of a specific gene or a group of genes. As to the pulmonary fibrosis-associated genes as well, there are reports that inhibition of the expression of those genes with siRNA was attempted (Patent Documents 1 to 4, Non Patent Documents 1 to 6).
However, the technologies reported to date have only exhibited inhibitory effect of an siRNA sequence on a disease-associated gene in experimental animals (mice and rats), while they have not sufficiently exhibited effects specifically on human genes. Further, concerning the inhibitory effect of siRNA, while the effects of siRNA at concentrations of 200 nM (Non Patent Document 1) and 20 to 500 nM (Non Patent Document 5) are exhibited, a nucleic acid molecule capable of inhibiting the expression of a pulmonary fibrosis-associated gene efficiently at a low concentration is not demonstrated.
Several tens of siRNAs targeting the TGF-β1 gene have been reported so far (Patent Documents 1 and 5 to 7). However, considering that the full-length TGF-β1 gene consists of 2346 bases (GenBank Accession No. NM_000660.3) and there are countless possible combinations of selecting an approximately 20-mer sequence out of the full-length gene, it is not easy to design an dsRNA or siRNA molecule capable of more efficiently inhibiting the expression of the gene from the combinations.